Distributed optical fiber sensing that enables simultaneous vibration and temperature monitoring is demonstrated in this work, which is achieved by implementing polarization optical time-domain reflectometry (P-OTDR) and Raman optical time-domain reflectometry (ROTDR) respectively through space-division multiplexed (SDM) configuration in different cores of a multicore fiber (MCF). Optimized system setup is obtained in the proposed hybrid system, where only one laser source is used, and the generated pulse is shared by the two reflectometers. Owing to the reason that separate interrogation fiber cores are used by the two reflectometers, the proposed hybrid system allows for simultaneous measurements of spontaneous Raman and Rayleigh scattering signals, thus it no longer suffers from the incompatible pump power levels issue, which results from different pump power demands, and hinders the implementation of the hybrid system in single mode fiber due to the fiber nonlinear effects imposed restriction. In addition, in order to extend the sensing range, we also explore the employment of wavelet transform denoising technique in the improvement of signal-to-noise ratio of ROTDR measurement. Eventually, simultaneous distributed vibration and temperature sensing in 5.76 km sensing range with 3 m spatial resolution and 0.57 ◦C temperature uncertainty has been demonstrated. The proposed hybrid sensing system has the outstanding advantage of high reliability, since the cross-sensitivity issue is effectively mitigated, which shows great potential for pipeline monitoring in oil and gas industry.

Introduction

The intelligent monitoring of environmental variation in the fields of civil, mining, and energy exploration has attracted ever increasing attention recently, where the environmental parameters (e.g. vibration, temperature and strain, etc.) are utilized to perceive the structure integrity and health condition, so that the safety and reliability of industrial operations can be enhanced notably. Specifically, simultaneous vibration and temperature sensing are highly desired for long range and distributed pipeline monitoring in oil and gas industry [1], [2], which can be used for intrusion detection and leakage detection, respectively. Distributed optical fiber sensing (DOFS) technique based on the measurements of backscattering signals turns out to be one of the most promising solutions for this application, thanks to its outstanding capability of long sensing range (tens of kilometers) and high spatial resolution (meter-scale) [3]. However, it is difficult to achieve simultaneous and discriminative measurement between vibration and temperature by using a single distributed fiber sensing technique, because most of the DOFS techniques are suffering from the cross-sensitivity issue among multiple parameters, which will decrease dramatically the reliability of measurements in real field deployment. One solution of this problem is to use two sensing techniques simultaneously, where one is used to monitor vibration, meanwhile the other one can be utilized to sense temperature.